1. Field of the Invention
The present invention relates to a voltage drive circuit, a voltage drive apparatus and a semiconductor-device testing apparatus. More particularly, the present invention relates to a voltage drive circuit generating voltage in accordance with an input signal, a voltage drive apparatus including the voltage drive circuit, and a semiconductor-device testing apparatus including the voltage drive circuit and the voltage drive apparatus.
2. Description of the Related Art
Among a test list for testing a semiconductor-device, there are a lot of test items related to an operational margin of the semiconductor-device, which is a device under test and will be simply referred to as a DUT hereinafter. In order to secure the performance conditions for the DUT that the DUT is operated under, which is a power voltage ranging between 3 volt.+-.10%, a semiconductor-device testing apparatus for testing the DUT needs to test the DUT using an input signal having same range of the voltage as the performance conditions of the DUT under the power voltage of that range. Considering an input signal A having a voltage of 3 volts, the semiconductor-device testing apparatus is required to output the input signal A with a maintained logic value, but shifted voltage to between 2.7 volts to 3.3 volts. Therefore, a voltage drive circuit that outputs the input signal A as varying voltage should be incorporated.
FIG. 1 shows a structure of a conventional voltage drive circuit 10. The input signal A is generated by a pulse generator P1 as a differential signal. The input signal A can take two values including a high value whose logic value is one, and a low value whose logic value is zero. When the input signal A is high, it means that the input signal A takes the high value, a positive output P1p from the pulse generator P1 becomes high and a negative output P1m becomes low. The positive output P1p of the pulse generator P1 is connected to a base of the transistor Q1, and the negative output P1m of the pulse generator P1 is connected to a base of the transistor Q2. Thus, when the input signal A is high, the transistor Q1 is switched on and the transistor Q2 is switched off. When, on the other hand, the input signal A is low, the transistor Q1 is switched off and the transistor Q2 is switched on. As the voltage drive circuit 10 is constructed such that either of the transistors Q1 and Q2 is switched on when the other of the transistors Q1 and Q2 is switched on, the current flowing in the voltage drive circuit 10 can be kept constant. Thus, the variation of the power voltage of the circuit 10 can be reduced and the circuit 10 can be operated at high-speed.
An emitter of the transistor Q1 and an emitter of the transistor Q2 are connected to a current drive circuit iG1 that generates a constant current i. A collector of the transistor Q1 and a collector of the transistor Q2 are connected to a constant voltage source VG1 respectively via the resistance R1 and the resistance R2. The constant voltage source VG1 generates a voltage VH. In this circuit 10, the signal is output from an output point 12 located between the resistance R2 and the transistor Q2, as an output voltage Vout. The output signal maintains its logic value, but has the output voltage Vout which is different from that of the input signal A.
In the above structure, the transistor Q2 is switched on when the input signal A is low, and the constant current i flows through the resistance R2. At this time, the output voltage Vout becomes Vout=VH-i*R2 (formula 1). On the other hand, the transistor Q2 is not switched on when the input signal A is high, and the constant current i flows through the transistor Q1. At this time, the output voltage Vout becomes Vout=VH (formula 2). As described above, the input signal A can take two values including high and low. Thus, the output voltage of the input signal A can be varied in accordance with either of the formulas 1 and 2 which are obtained depending on the values of the input signal A.
However, due to recent developments in semiconductor techniques, performances of semiconductor-devices are highly advanced. Thus, the semiconductor-device testing apparatus needs to have precise performance.
As for the circuit 10 shown in FIG. 1, for example, a base current ib is actually input to the base of the transistor Q2 and thus the actual output voltage Vout does not match the value obtained in accordance with the above formula 1. The base current is a nonlinear function depending on voltage between the collector and the emitter, and a collector current. Furthermore, the base current can easily be affected by errors caused during manufacturing. Thus, it is difficult to adjust the circuit by the base current ib taking into consideration.